[18.09] On Variations in the Peak Luminosity of Type Ia Supernovae

We explore the idea that the observed variation in the peak
luminosities of Type Ia supernovae originates in part from a
scatter in metallicity of the main-sequence stars that
become white dwarfs. For any explosion model that reaches
nuclear statistical equilibrium outside the innermost
0.2\,M\odot, the mass of 56\mathrm{Ni} produced
depends linearly on the initial metallicity of the
progenitor. Detailed post-processing of W7 models confirms
this linear dependence. We calculate that the observed
scatter in the initial CNO abundances (from one-third to
thrice solar) induces a 25% variation in the mass of
56\mathrm{Ni} ejected by Type Ia supernova, sufficient
to account for about a 0.2 magnitude variation in the peak
V-band luminosity. This scatter in metallicity is present
out to the limiting redshifts of current observations (Z <
1). Sedimentation of the 22\mathrm{Ne} can further
reduce the amount of 56\mathrm{Ni} produced, making the
maximum variation < 50% in the 56\mathrm{Ni} mass
(0.4 magnitude in V). Higher metallicity systems produce
more 54\mathrm{Fe} and 58\mathrm{Ni}; this may
imply that the 54\mathrm{Fe}/56\mathrm{Fe} abundance
ratio has increased over the past ~5\,\mathrm{Gyr}.
This work is supported by the Department of Energy under
Grant No.~B341495 to the Center for Astrophysical
Thermonuclear Flashes and Grant No.~DE-FG02-91ER40606 in
Nuclear Physics and Astrophysics at the University of
Chicago.